Radiation activated self-cleaning coatings, such as photo-catalytic coatings, can be used for many applications. For example, such a coating formed on an exterior surface of a building is able to kill bacteria or oxidize organic pollutants.
A known type of self-cleaning coating is one that contains a metal oxide such as titanium dioxide (TiO2). When the coating is exposed to an electromagnetic radiation having energy larger than the band gap (between the conduction band and the valence band) of the metal oxide, electrons in the valence band can be excited to the conduction band, leaving a positively charged hole in the valence band. The photoexcited holes have the ability to cause oxidation reactions and the electrons have the ability to cause reduction reactions. Unless they re-combine among themselves, these photoexcited holes and electrons tend to move to the surface to induce oxidation-reduction. The chemical species in their vicinity will therefore be either oxidized or reduced. For example, a hole may oxidize a water molecule (H2O) to yield an OH; and an electron may reduce oxygen to a superoxide anion (O2−) or a hydrogen peroxide (H2O2). As reactive oxygen (OH.H2O2 and O2−) have very strong reactivity, they will break down large organic pollutants, completely mineralizing most organic compounds (including bacteria), and leaving carbon dioxide and water as products. The resulting products can be easily washed away. With rain-wash, this type of coatings exhibit self-cleaning cleaning effects.
A radiation activated surface can be utilized in many applications such as anti-bacteria, anti-fogging, deodorization and water purification applications.
The self-cleaning material such as TiO2 particles can be deposited directly on many inorganic substrates because these substrates, such as tiles and glasses, are resistant to photochemical reaction from the photo-catalytic coating. Typically, the TiO2 particles are deposited on the substrates and are then sintered at temperatures of several hundreds of degrees Celsius. A disadvantage of such a technique is the requirement to heat to very high temperatures.
The TiO2 particles can also be immobilized on the substrate on top of an under layer, or a binder. Example inorganic under layers or binders include water glass, silicate coating, silicone rubber and fluorinated polymer. Inorganic binders can be generated by hydrolysis of metal alkoxide precursors, for example, the hydrolysis of tetra ethoxyl silicate can produce silica binder. Example organic binders include polytetrafluoroethylene (PTFE), silicon resin, acrylate resin and melamine resin.
The use of under layers or binders can be advantageous in cases where the substrates, such as some polymeric substrates, can be damaged by the reactions activated by radiation, or where the self-cleaning material does not adhere well to the substrate directly.
The conventional techniques using under layers or binders, however, also have certain drawbacks.
Some inorganic binders and under layers have limited critical (defect-free) thickness especially when their precursors have four hydrolysable functional groups. Dip and spin coating are typically the only suitable methods for depositing these binders or under layers on the substrate. When the precursors for the binder or under layer contain less than four functional groups, the adhesion to the substrate is poor. Further, the resulting coating generally needs to be cured at temperatures higher than 200° C. Organic substrates may be deformed or damaged at such high temperatures.
A disadvantage of conventional organic binders is that they tend to reduce the photo-catalytic activity of the photo-catalytic particles. Another problem is that the organic binder can be gradually oxidized if the coating is exposed to radiation such as sun light for a long period of time.
Accordingly, there is a need for improved processes and materials for forming radiation activated self-cleaning coatings.